In Spanish, the use of "tú" (the informal "you") typically applies to people with whom you have a close or familiar relationship. Here's how it applies to the people you mentioned:
- Your sister - You would likely use "tú" with your sister.
- Your friend - You would also use "tú" with your friend.
- Your friend's mom - You would typically use "usted" (the formal "you") with your friend's mom, unless you have a very close relationship with her.
- Your pet - You would use "tú" when talking to your pet, as it's an informal context.
So, you are likely to use "tú" with your sister, your friend, and your pet.